This invention relates generally to sensors of parameters, particularly to a class of sensors that from which values of the measured parameters are communicated by a radiation link, without the need for wires or other physical link to the sensors.
Applications of sensors to measure various physical, electrical and chemical parameters are numerous. Each such sensor is, to the extent possible, adapted to optimally perform in a specific application or range of applications to measure one or more parameters. This includes making a sensor of a particular parameter as insensitive as practical to variations of other parameters in the environment in which the sensor is placed. This also includes making the sensor of a configuration and set of materials to minimize any effect on the environment in which it is placed that could alter the parameter being measured. Further, a method of communicating a signal related to the measured parameter between the sensor and a measuring device is also chosen to be consistent with the application and the environment.
One such sensor application measures a parameter, such as temperature, on a surface of an object. One specific class of surface temperature measuring devices includes indicators that change in a visually perceptive manner as a function of temperature. Such sensors are attached to the surface being measured and the temperature dependent visual indications are optically observed. In one class of such sensors, an array of a plurality of indicators that individually change color at different temperature thresholds is included, thus allowing the temperature of the surface to be observed by noting which of the indicators changed color and which did not. This type of sensor most commonly uses indicators that are not reversible, thereby to measure a peak temperature reached by the surface during a measurement. The peak temperature indicators are made from a powder material having a sharp melting point that is painted or printed onto a background of a dark or other contrasting color. When an indicator is raised to a threshold temperature of its melting point, the normally reflective powder, usually white, melts and is absorbed by the background material. The color of an indicator whose threshold melting temperature has been exceeded changes from white to dark. This type of peak temperature sensor is available from several manufacturers in the form of labels that are attached to a surface whose temperature is to be measured. The manufacturers include Wahl Instruments, Inc. of Culver City, Calif. Tempil, Inc. of South Plainfield, N.J, and Reatec AG of Dxc3xcbendorf, Switzerland. Another type of available indicator in the same form changes its visual state when its temperature, integrated over a period of time (time-temperature integral), exceeds a designated threshold.
Such a label includes a number of indicators that are each formed of a quantity of material, sealed by a transparent plastic layer, that changes color at a different temperature in a range of temperature. The threshold temperatures are usually marked on the label adjacent the indicators. The temperature of the surface is determined by observing the indicator with the highest temperature marking that changes its color. The surface temperature is thus in between the threshold temperature of that indicator and that of an indicator having the next higher threshold temperature that remains with its color unchanged.
One application of these surface temperature measuring labels is to measure the temperature of semiconductor wafers during processing. One or more labels are attached to a surface of a semiconductor wafer or other conductive substrate prior to being placed into a processing chamber where it is cycled through a range of temperatures with which semiconductor wafers are normally processed therein. After being cycled through that temperature range, the substrate is removed from the processing chamber and the temperature of the wafer is determined by observing by the label(s), a peak temperature if their indicators are the non-reversible type.
A sensor of one or more parameters including, but not limited to, temperature, includes at least one indicator encapsulated within a rigid enclosure in order to protect the indicator from pressure, particles and other elements of the environment in which the sensor is to be used. The sensor is made for immersion in the environment in which the parameter is to be measured, made for attachment to a surface to measure the parameter, or one or more sensors are formed at different locations across a surface as an integral part of it. If it is temperature that is being sensed, the enclosure is thermally conductive in order to minimize any vertical and lateral temperature gradients across the sensor during temperature measurement. The enclosure is made to allow penetration therethrough by electromagnetic radiation between the indicator and an observer or measuring instrument positioned a distance from the sensor. If the indicator change due to the changing parameter is visual, at least an area of the sensor enclosure is made transparent to visible electromagnetic radiation.
In a specific form of a temperature sensor, the sensor is characterized by an observable passive optical property changing as a function of its temperature. A plurality of indicators of specific temperatures, such provided by adhering the above-described commercially available labels within the enclosure or by painting the indicator material on an inside of the enclosure, may be utilized. If these indicators are positioned in a plurality of compartments formed between a bottom substrate and a cover sealed thereto that together form the enclosure, the temperature gradient across the enclosure between the substrate and cover is minimized, which improves the accuracy of the measurements. The window of such a two part enclosure is typically made to be optically transparent in order to allow the indicators to be seen through the cover.
An important use of a temperature sensor in one of the forms summarized above is in the measurement of the temperature to which semiconductor wafers are subjected in integrated circuit processing equipment. It is further often desired to measure the temperature distribution across a wafer within a processing chamber of such equipment. A semiconductor wafer or other similarly thermally conductive substrate that has a number of the sensors positioned at various locations across one of its surfaces may be positioned within the processing chamber while the equipment is operated through at least a portion of a processing cycle where the temperature of the wafer is important. The sensor(s) on the substrate may be monitored during the processing cycle, if a window exists into the chamber, or a, peak temperature that each of the sensors reaches may be observed upon removal of the test substrate from the chamber after the processing is complete. Indicators of the amount of time that a sensor was at elevated temperatures (time-temperature integrals) may be used instead of, or in addition to, the peak temperature indicators. This allows the equipment to be adjusted if a desired temperature is not reached, exceeded or maintained, or in order to modify the temperature distribution across the substrate. The rigid structure of the sensor enclosure allows temperature to be measured in processing chambers that reach a very low pressure and/or that utilize a plasma that impacts the sensor enclosure with high energy ions, with minimal effect on the sensor indicators, thereby improving the accuracy of the temperature measurement in such an environment.
According to another aspect of the present invention, an optical reader of a sensor having an array of binary parameter indicators with different thresholds, such as the peak and/or time-integral temperature indicators described above, compares an imaged parameter dependent indicator pattern with a library of patterns in order to automatically determine and display the sensor peak temperature and/or duration. In the case of several such sensors distributed across a surface to measure its thermal distribution, for example, the temperatures and/or temperature durations of the various locations of the sensors are displayed on an image of the surface. Other types of displays, such as a contour thermal map, may alternatively be provided.